1. Field of the Invention
This invention pertains to methods and apparatus for removing water-soluble impurities from water-insoluble particles. More particularly, this invention pertains to methods and apparatus that use filtration for purifying a fluid aqueous suspension may be containing water-soluble impurities.
2. Description of the Relevant Art
Water-insoluble particles sometimes have-uses which require the removal of water-soluble impurities. Such uses include use as a mordant in dyeing, an ion-exchanger, a filtering medium, a chromatography substrate, a printing ink, an antiperspirant, a dentifrice, a tablet filler, and others. For example, aluminum hydrate, also known as aluminum hydroxide, hydrated alumina, or aluminum trihydrate, may be used as a mordant in dyeing, an adsorbent, an emulsifier, an ion-exchanger, a filtering medium, an antacid, and an antihyperphosphatemic. Aluminum hydrate is also used in chromatography; in the manufacture of glass, fire clay, paper, pottery, printing inks, lubricating compositions, and detergents; for waterproofing fabrics; and in antiperspirants and dentifrices, among other applications.
Alumina hydrate and aluminum chloride are useful for making "lakes". "Lakes" are color additives prepared by combining a soluble dye with an insoluble substrate. Lakes made from aluminum compounds are generally thought to be coordination compounds of the aluminum compound with the dye. An aluminum lake is conventionally prepared by adding a base such as sodium carbonate or sodium hydroxide to a solution of aluminum sulfate to precipitate aluminum hydrate. That process generates sodium sulfate as a water-soluble byproduct, which is removed by pressing a filter cake and washing the filter cake in the filter press. A solution of colorant is added to the resulting slurry, which is acidified with hydrochloric acid to convert the colorant to an aluminum salt, which then absorbs onto the surface of the aluminum hydrate to form a lake. In that step, sodium chloride is formed as a water-soluble byproduct.
A lake may also be formed by adding a dye to an aluminum hydrate formed by adding aluminum chloride to a solution of sodium bicarbonate as described in U.S. Pat. No. 3,909,284, incorporated herein by reference. According to that patent, a dye dissolved in water is added to the resulting aluminum hydrate suspension and then a thin stream of aluminum chloride solution is added until the pH is between 4.1 to 4.3 to effect laking. An advantage of this process is-that it does not generate sulfate salts, but it still generates chloride salts as an intermediate byproduct.
The slurry formed by either lake making process is filtered and the filter cake is then washed, dried and ground to the appropriate fineness. The product is marketed as is, mixed with other lakes, mixed with approved diluents, or dispersed in edible vehicles or other media that make the mixtures appropriate for printing food wrappings, marking capsules, coloring health and beauty products that come in contact with the skin, coloring products intended for human consumption, etc.
When the colorants used to make lakes are intended for human contact or consumption, they are generally required to be "certified" by an appropriate government agency. In the U.S., that agency is the U.S. Food & Drug Administration (the FDA). U.S. certified colorants are generally referred to as FD&C (Foods, Drugs and Cosmetics) and D&C (Drugs and Cosmetics) colorants. Other entities which certify colorants include the European Economic Community (E.E.C), the World. Health Organization (W.H.O.), and most developed countries. Alternative substrates for making "lakes" approved in some countries, include blanc fixe, gloss white, clay, titanium dioxide, zinc oxide, talc, rosin, aluminum benzoate, calcium carbonate, and combinations thereof.
The need for removal of the water-soluble salts generated at each step of the "lake" manufacturing process is a serious problem, because copious amounts of water has been required to wash sodium sulfate from the aluminum hydrate and wash sodium chloride and non-absorbed dye from the lake. The presence of sulfates interferes with absorption of the dye on the hydrate base and the presence of sodium chloride and excess dye in the lake can reduce water-resistance and resistance to color bleed.
The current industry practice for purifying such water-insoluble substrates and "lakes" prepared from them is a batch process that utilizes a plate and frame filter press. A fluid aqueous suspension of particles is dewatered by passing it through a filter press. The filter press cake is then washed with purified water while in the press until the level of dissolved salts in the filtrate, as measured by a conductivity meter, is reduced to a satisfactory level. The inventors have found that a disadvantage of this process is that the filter cake often forms "channels" or "short circuits" for the passage of water through the cake during washing, preventing the water from effectively cleansing the entire filter cake. This results in an ineffective wash and a lower quality batch containing a higher content of impurities when it is reslurried. Refiltering the reslurried filter cake requires additional time and labor, increasing production costs.
A need therefore exists for an improved process for purifying fluid aqueous suspensions of water-insoluble particles containing water-soluble impurities. Specifically, a process is needed which is capable of producing commercial amounts of high purity water-insoluble particles in less time and/or labor than conventional methods. Such a process should be as economical or more economical than conventional methods. The process should use as little water as possible to achieve a given purity level to reduce the cost of generating purified wash water and, at the same time, reduce the cost of safely disposing of or recycling impurity-containing filtrate. Furthermore, such a process should be reliable and require a low degree of maintenance. These and other objectives are attained by the present invention.